© 2008 by the Loma Linda University Press. All rights reserved.

May not be copied or reused, other than for personal use, without express written

permission of the publisher.

Click here for details about the volume and how to order it,

or send e-mail to rattlesnakes@llu.edu

Reprinted from

353

__________________________2 Correspondence e-mail: edugan1@hotmail.com© 2008 Loma Linda University Press

Pp. 353-364 in W. K. Hayes, K. R. Beaman, M. D. Cardwell, and S. P. Bush (eds.), The Biology of Rattlesnakes. Loma Linda University Press, Loma Linda, California.

Home Range Size, Movements, and Mating Phenology of Sympatric Red Diamond (Crotalus ruber) and Southern Pacific

(C. oreganus helleri) Rattlesnakes in Southern California

Eric A. Dugan1,2, Alex Figueroa1, and William K. Hayes1

1Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, California 92350 USA

Abstract.—Although many sympatric snake species partition food to reduce interspecific competition, rattlesnakes and other vipers, like most vertebrates, typically partition the habitat. To evaluate this generality, we used radiotelemetry to study the home range sizes, movements, and mating phenology of sympatric adult male Red Diamond (Crotalus ruber) and Southern Pacific (C. oreganus helleri) Rattlesnakes in a coastal valley of southern California. Mean home range sizes and mean daily movements were substantially greater in C. o. helleri than in C. ruber. Both species occupied much larger home ranges during the two active seasons (March-November) compared to the winter season. Mating seasons differed between the two species, with C. ruber engaging in accompaniment, courtship, and copulation exclusively in the spring and C. o. helleri exhibiting reproductive behavior during both late summer/fall and spring. Annual movements by C. o. helleri spiked in both spring and late summer/fall, whereas movements of C. ruber spiked also in spring but less dramatically in late summer/fall. Spatial use and movement differences between the two species likely resulted from distribution of preferred habitats at the study site (limited Opuntia cactus patches for C. ruber, widespread non-native grassland and riparian habitats for C. o. helleri) and differences in mating phenology (C. o. helleri males searching for mates in two seasons versus one for C. ruber). Thus, these two species, like other sympatric rattlesnakes and vipers studied to date, appear to partition habitat. However, unlike other sympatric rattlesnakes studied to date, C. ruber and C. o. helleri also differ in their use of spatial and temporal resources, though probably not as a direct result of competitive mechanisms.

Introduction

Niche theory predicts that closely-related sympatric species should partition resources to avoid or reduce inter-specific competition (Pianka, 1981; Walter, 1991). Niche separation can be achieved via differences in food (type or size) or in use of spatial (macrohabitat or microhabitat), temporal (diel or seasonal activity patterns), or thermal resources (Schoener, 1974; Saint Girons, 1978). Snakes have been considered atypical among vertebrates because sympatric species usually partition food rather than habitat (Toft, 1985; Vitt, 1987; Luiselli, 2006a). However, sympat-ric viper species apparently adhere to the typical vertebrate pattern, primarily partitioning habitat (Luiselli, 2006a,b; Luiselli et al., 2007).

Rattlesnakes (genera Crotalus and Sistrurus) represent an ideal group for exploring niche separation in vipers. The ranges of many species often overlap broadly, particularly in the arid and semi-arid regions of southwest North America, where species richness is highest (Klauber, 1972; Beaman and Hayes, this volume). The few studies of this group sup-port the view that sympatric rattlesnake species primarily partition habitat. Pough (1966), Reynolds and Scott (1982), and Mendelson and Jennings (1992) demonstrated differ-ences in habitat use of Crotalus atrox, C. molossus, and C. scutulatus in the Sonoran Desert of southeast Arizona,

southwest New Mexico, and northern Chihuahua, Mexico. Beck (1995) determined that sympatric C. atrox, C. molos-sus, and C. tigris in southeast Arizona use different habi-tats, but otherwise exhibit similar home range sizes, activity patterns, thermal ecology, and annual food intake. Waldron et al. (2006b) found that sympatric C. adamanteus and C. horridus in coastal South Carolina prefer different habitats, but occupy similar home range sizes. Steen et al. (2007) also showed that sympatric C. adamanteus and C. horri-dus in coastal Georgia use different habitats. Reinert (1984) determined that C. horridus and another sympatric viper, Agkistrodon contortrix mokeson, prefer different habitats in eastern Pennsylvania.

Differences in space use and movements by sympatric rattlesnakes could be expected for many reasons apart from competitive mechanisms. The distribution and abundance of critical resources greatly influence the movement patterns of snakes (Gregory et al., 1987). Critical resources for rat-tlesnakes include required habitat, food, access to potential mates, protection from predators, and suitable thermoregu-lation and overwintering sites (e.g., Reinert and Zappalorti, 1988; Secor, 1992; Duvall and Schuett, 1997). Temporal variation in resources and needs also influences movements by snakes (Seigel and Pilgrim, 2002). Some crotaline snakes, for example, mate only in spring, some mate only in late summer/fall, and others exhibit a bimodal mating phenol-ogy, with copulations occurring during both late summer/fall and spring (i.e., interrupted by hibernation; Aldridge and Duvall, 2002; Schuett et al., 2002). During mating season,

354 E. A. Dugan, A. Figueroa, and W. K. Hayes

males frequently undertake extensive searches for mates (e.g., Duvall et al., 1992; Secor, 1992; Aldridge and Brown, 1995; Prival et al., 2002; Sealy, 2002; Ashton, 2003; Mar-shall et. al, 2006; Waldron et al., 2006a; Jellen et al., 2007; Cardwell, 2008; Brown et al., this volume; Goode et al., this volume). Rattlesnake activity also shifts dramatically with seasonal changes in temperature and rainfall (e.g., Prival et al., 2002; Goode et al., this volume). Thus, differences in space use and movements by sympatric rattlesnakes could result from differences in preferred habitats and species-specific eco-physiological constraints.

The Red Diamond Rattlesnake (C. ruber) is a large-bodied pitviper distributed from southern California south-ward throughout the Baja peninsula and several of its asso-ciated islands (Grismer, 2002). Crotalus ruber can be found from the desert slopes to the coastline, and preys predomi-nately on rodents (Klauber, 1972; Dugan, unpubl. data). The Red Diamond rattlesnake is currently listed as a species of special concern by the State of California (Jennings and Hayes, 1994). To date, no published accounts exist detail-ing the biology of this species (Beaman and Dugan, 2006); however, recent research has begun to address this void in our knowledge (Greenberg, 2002; Brown et al, this volume; Halama et al., this volume).

The Southern Pacific Rattlesnake (C. oreganus hell-eri; Ashton and De Queiroz, 2001) is another large-bodied pitviper, distributed from central California southward into northern Baja (Grismer, 2002). This snake is considered a habitat generalist (Stebbins, 2003), as it inhabits a wide range of habitats from montane coniferous forests to coastal sage scrub along the coast. The Western Rattlesnake com-plex (C. viridis + C. oreganus + C. cerberus) is one of the most extensively studied group of snakes (Diller and Wal-lace, 2002; Beaman and Hayes, this volume). However, no quantitative data have been published on movements and habitat use of C. o. helleri.

The close phylogenetic relationship and extensive overlap of the ranges, habitat, and general life histories makes these two species ideal candidates for studies of niche separation and interspecific variation in natural his-tory. In California, coastal populations of both species co-exist in a rapidly-fragmenting environment, placing many remaining populations in danger of genetic isolation and local extirpation (Halama et al., this volume). Accordingly, we need a better understanding of the ecology of these two broadly sympatric species, not just for behavioral and eco-logical theory, but also for conservation concerns. The data presented herein address the following questions as they relate to potential niche separation. 1) Do home ranges of sympatric C. ruber and C. o. helleri differ in area and by season? 2) Do movements of sympatric C. ruber and C. o. helleri differ in magnitude and by season? And finally, 3) does the phenology of reproductive activity differ between C. ruber and C. o. helleri? Additional data collected during this study on habitat use, diet, and survival will be presented in a separate manuscript.

Materials and Methods

Study site.—The study site encompassed ca. 30 ha in the southeastern portion of Chino Hills State Park (CHSP), California (33º54' N, 117º42' W). A north-south running canyon with a small semi-perennial creek dominates the site’s topography. The CHSP system comprises ca. 5,039 ha in Los Angeles, Orange, Riverside, and San Bernardino counties, and ranges in elevation from 131-543 m (Keller, 1992; Goodman, 1997; http://www.stateparks.com/chino_hills.html).

We identified five major habitats at the study site: cac-tus, coastal sage scrub (CSS), non-native grassland, riparian, and oak woodland. Cactus patches of Coastal Pricklypear (Opuntia littoralis) were found exclusively on south-facing slopes and represented the habitat with the smallest dis-tribution at our site. The distribution of CSS was patchy, primarily on south-facing slopes. Non-native grassland was the most widespread, occurring on both hillsides and in canyon bottoms. The riparian system was largely confined to the creek channel, but penetrated into damper drainages of the grassland. Oak woodland was found primarily on north-facing slopes, but was not used by any of the teleme-tered snakes in this study. The study site was devoid of rock outcroppings, as the soil is predominately clay. Sampson (1985) provided further details on relative abundance and species composition of these plant communities.

The site experiences a Mediterranean climate. Aver-age annual precipitation ranges from 35-46 cm, with the majority of rainfall occurring during the winter and spring months (Sampson, 1985). Winters are mild (average daily low in January = 5.6°C) and summers hot and dry (average daily high in July = 31.7 °C), with temperatures occasion-ally exceeding 38°C.

Radio-telemetry.—We began collecting, marking, and implanting snakes with radio-transmitters in March 2003. Individuals were tracked from March 2003 through De-cember 2004. Visual searches and road surveys were used to obtain snakes of both species. Snakes were anesthetized with sevoflurane (Halocarbon Products Corp., River Edge, New Jersey, USA) while restrained within clear plastic tubes (Hardy and Greene, 1999) to allow collection of various measurement data. We recorded snout-vent length (SVL), total length, mass, rattle number, number of subcau-dal scales, and sex for each snake. Adult snakes were sexed using Neosporin-lubricated sexing probes. All individuals were marked with a passive integrative transponder (PIT) tag (AVID Identification Systems, Inc., Norco, California, USA). PIT tags allowed us to permanently identify indi-viduals as part of a long-term mark-recapture effort.

We used SI-2T temperature-sensitive transmitters (Holohil Systems Ltd., Ontario, Canada) to monitor up to six males of each species simultaneously. Transmitters weighed 9 g and always represented <5% of an individual’s body mass (Hardy and Greene, 1999). Surgical procedures followed the guidelines and methods described by Reinert

355Ecology of Red Diamond and Southern Pacific Rattlesnakes

and Cundall (1982) and Hardy and Greene (1999). Snakes were released at their collection site 24-36 h post-surgery. Minimizing time in captivity has been shown to increase post-surgical survival (Hardy and Greene, 1999). Sampling effort varied seasonally as snake activity patterns changed. Individuals were located 1-4 times/wk throughout the active season (March-November) and less often (bi-monthly) dur-ing the winter period (December-February). Telemetered snakes were relocated using a Telonics TR2 receiver (Te-lonics, Mesa, Arizona, USA) and a hand-held four-element yagi antenna. Upon each relocation, we visually located each snake if possible and recorded the universal transverse mercator (UTM) coordinates with a handheld GPS unit (Garmin GPS Plus III; Garmin Ltd., George Town, Cayman Islands). When snakes were located in dense, impenetrable cactus patches, coordinates were taken at the closest loca-tion possible (within 5 m of presumed location).

Data presented herein were collected from nine adult male C. o. helleri (84-103 cm SVL) and seven adult male C. ruber (98-156 cm SVL). Snakes were lost (predation, trans-mitter battery failure) or added opportunistically throughout the study, resulting in variable tracking periods (202-905 d) for different individuals.

Home range size and movements.—Based on loca-tion data, we computed two estimates of home range size, one estimate for autocorrelation, and one movement vari-able (see below). We compared each of these dependent measures for the two species and for three seasons: active season (March-November) 2003, winter season (Decem-ber-February) 2003-2004, and active season (March-No-vember) 2004.

We computed seasonal home range sizes using both minimum convex polygons (MCP) and fixed-kernel (FK) methods. We used Calhome 1.0 (Kie et al., 1994) and Ho-meRanger 1.5 (Hovey, 1999) to obtain 100% MCP and 95% FK, respectively. We used all fixes obtained from each individual for our analyses and software defaults, includ-ing least-squares cross-validation as the smoothing factor for FK. Although FK estimates are increasingly preferred (Powell, 2000), MCP estimates continue to be reported and are more available in the literature for comparisons between studies. Both estimates are sensitive to sample size, with MCP increasing asymptotically and FK decreasing asymp-totically with increasing number of fixes (Seaman et al., 1999). Opinions remain contentious as to which estimate (MCP vs. FK) performs better (e.g., Row and Blouin-Dem-ers, 2006; Laver and Kelly, 2008). HomeRanger also com-puted autocorrelation (t2/r2) for each snake, a measure of temporal independence between successive fixes. When au-tocorrelation exists (t2/r2 < 2), the distance moved between consecutive observations decreases, resulting in underesti-mates of FK (Swihart and Slade, 1985).

We considered movements at three time levels: daily, monthly, and seasonal. We used the mean distance moved per day as our fundamental unit of measurement for indi-vidual snakes. This was computed from the distance moved

between each two consecutive fixes divided by the number of days between the two fixes (Gregory et al., 1987). To obtain mean daily movements per month, we calculated the mean of all such measurements procured within a given month for each snake. To avoid bias from uneven sampling across months, mean daily movements per season were computed as the average of all monthly estimates (rather than the mean from all fixes) for those months within a given season.

Mating phenology.—Data were collected opportunis-tically from both telemetered and non-telemetered snakes throughout the duration of the study. We recorded two types of sexual activities: accompaniment (pairs coiled next to or within 0.5 m of each other, often with males chin-rubbing and/or pursuing females in courtship; c.f. Duvall et al., 1992; Duvall and Schuett, 1997) and copulations. Non-sexual accompaniment during the winter months (overwin-tering at same site, November-February) was excluded. We also gleaned additional observations from the existing lit-erature on C. ruber and C. o. helleri.

Statistical analyses.—Our primary interest was to ex-amine the effects of species and season on spatial use and movements. For home range and movement data, we sub-jected each of the four dependent variables (100% MCP and 95% FK estimates of home range size, mean daily move-ments, and autocorrelation values) to three analyses.

First, to evaluate species and season simultaneous-ly, we conducted 2 × 3 analyses of variance (ANOVAs; Mertler and Vannatta, 2004), treating species (two levels) as a between-subjects factor and season (three levels) as a within-subjects factor. Because some telemetered snakes were lost during the study (one of the initial six C. ruber became ill; three of the initial six C. o. helleri were pre-dated) and then replaced by others, only five C. ruber and three C. o. helleri were tracked continuously over the three seasons, rendering a small sample size. Effect sizes from the ANOVAs were obtained as partial η2 values (Cohen, 1988), indicating the approximate proportion of variance in the dependent variable explained by each independent vari-able or interaction, with values >0.25 generally considered large. When multiple effect sizes within a model summed to >1, we adjusted the values by dividing each partial η2 by the sum of all partial η2 values. Second, we conducted independent-samples t-tests to compare the two species for each of the three seasons, and computed Cohen’s d (Cohen, 1988) for effect sizes using pooled standard deviation. Co-hen’s d values >0.8 are generally considered large effects. This within-season paired-comparison approach utilized all of the available subjects, increasing the sample size for the two active seasons (N = 6 for each species). Third, we used Pearson correlation coefficients of determination (r2)to ex-amine associations among the home range estimators (MCP and FK), number of fixes, autocorrelation, and movements.

Data used in these analyses were examined to deter-mine whether parametric assumptions were met, and the fit was found to be acceptable in all cases. We also conducted

356 E. A. Dugan, A. Figueroa, and W. K. Hayes

non-parametric equivalents of t-tests and Pearson coeffi-cients, but because the conclusions were identical, we re-port only the parametric outcomes.

Frequency data for sexual behavior were compared be-tween the two species for the spring and late summer/fall seasons. The resulting 2 × 2 contingency table had too few cells with expected frequency ≥5 to use Chi-square tests. Thus, we evaluated the strength of asymmetry between spe-cies and seasons using Cramer’s V (Conover, 1999).

Analyses were conducted using SPSS 12.0 for Win-dows (SPSS, 2003) with alpha set at 0.05.

Results

Home range size.—The two estimates of home range size (100% MCP and 95% FK) are summarized in Table 1 for each snake during each of the three seasons (the FK estimate for CR2 in the active season 2004 was deleted from analysis as an obvious outlier). Individual home range es-timates varied from 0.15-23.02 ha during the active season and from 0.01-0.26 ha during the winter. Comparisons be-tween species and seasons are depicted in Fig. 1. The main effect of species in the ANOVA model was not significant for either MCP or FK (both P ≥ 0.16 and partial η2 ≤ 0.32;

Table 2). However, the main effect of season was signifi-cant for both MCP and FK (P = 0.041 and 0.033, respec-tively), with each measure having a large effect size (partial η2 = 0.47 and 0.43, respectively; Table 2). As expected, the snakes occupied much smaller home ranges during the win-ter, with C. ruber and C. o. helleri averaging 3.6% and 1.8% of their active-season home ranges, respectively (computed using species means in Table 1, with active-season home range calculated as the average for the two estimators, and averaged again for the two active seasons). No interactions between season and species were detected (both P > 0.20 and partial η2 = 0.25; Table 2). Given the relatively large effect sizes (Cohen, 1988) for species and for interaction of species × season, larger samples may well have yielded significance. When the two species were compared for each season by t-tests, no species differences were detected in any season (all P ≥ 0.056). However, the effect sizes (Cohen’s d) were large (active 2003: MCP, d = 1.25, FK, d = 0.85; winter 2003-2004: MCP, d = 0.14, FK, d = 0.58; active 2004: MCP, d = 0.76, FK, d = 0.85), especially during the active seasons, further suggesting that C. o helleri occupied larger home ranges than C. ruber. Mean home range estimates were two- to five-fold greater for C. o. helleri (Table 1).

Table 1. Summary of number of locations (fixes), home range estimates (100% minimum convex polygon, MCP; 95% fixed kernel, FK; in hectares), mean daily movements (m/d), and Schoener’s autocorrelation (t2/r2) per each of three seasons for individual male Red Diamond (Crotalus ruber, CR) and Southern Pacific (C. oreganus helleri, CH) Rattlesnakes.

Species- Active Season 2003 Winter Season 2003-2004 Active Season 2004 Snake Fixes MCP FK m/d t2/r2 Fixes MCP FK m/d t2/r2 Fixes MCP FK m/d t2/r2________________________________________________________________________________________________________________ CR1 29 4.05 7.53 9.42 1.02 11 0.17 0.20 0.85 2.59 29 3.72 1.62 5.40 0.62 CR2 32 3.80 1.77 5.45 0.67 8 0.01 0.05 1.07 2.76 6 4.50 127.02a 16.46 0.89 CR8 27 1.26 0.73 6.02 1.89 6 0.06 0.03 1.58 1.21 7 0.63 0.52 26.06 2.46 CR12 23 2.36 1.57 6.22 1.46 10 0.10 0.02 2.21 2.67 40 3.58 1.11 9.50 1.52 CR13 23 1.58 0.60 5.76 1.95 9 0.02 0.03 1.04 0.87 39 1.96 1.16 11.96 1.45 CR19 23 0.34 0.15 2.93 1.59 - - - - - - - - - - CR36 - - - - - - - - - - 13 0.93 0.61 5.39 1.47________________________________________________________________________________________________________________

Mean 26.2 2.23 2.06 5.97 1.43 8.8 0.07 0.07 1.35 2.02 22.3 2.55 1.00 12.46 1.40 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1 SE 1.6 0.60 1.12 0.85 0.20 0.9 0.03 0.03 0.25 0.40 6.4 0.66 0.20 3.21 0.26________________________________________________________________________________________________________________ CH1 10 3.22 2.90 12.18 0.86 - - - - - - - - - - CH2 27 4.82 0.92 13.23 2.07 - - - - - - - - - - CH3 34 3.46 2.14 14.13 1.32 - - - - - - - - - - CH5 25 1.98 2.81 4.70 1.16 10 0.01 0.06 0.88 2.80 10 6.10 0.22 25.83 1.94 CH7 23 8.53 15.28 21.79 1.80 10 0.19 0.26 3.69 1.06 37 23.02 9.73 17.39 1.26 CH8 24 6.13 12.38 34.70 1.71 10 0.05 0.05 3.40 1.47 12 4.45 16.39 37.18 2.47 CH33 - - - - - - - - - - 8 0.90 2.60 21.78 1.44 CH39 - - - - - - - - - - 19 3.54 0.17 27.06 0.80 CH42 - - - - - - - - - - 32 3.70 0.84 14.28 1.49________________________________________________________________________________________________________________ Mean 23.8 4.69 6.07 16.79 1.49 10.0 0.08 0.12 2.66 1.78 19.7 6.95 5.00 23.92 1.57 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1 SE 3.2 0.96 2.50 4.21 0.18 0.0 0.05 0.07 0.89 0.53 5.0 3.29 2.72 3.31 0.24 a Regarded as an outlier and excluded from analyses.

357Ecology of Red Diamond and Southern Pacific Rattlesnakes

The 100% MCP and 95% FK estimates from individ-ual snakes were positively correlated during the first two seasons (active 2003: N = 12, r2 = 0.74, P < 0.001; winter 2003-2004: N = 8, r2 = 0.70, P = 0.009), but not during the third (active 2004: N = 11, r2 = 0.22, P = 0.15). Although individual MCP estimates were usually larger than cor-responding FK estimates during the active season (73.9% of 23 estimates), the FK estimates were more often larger during the inactive season (62.5% of 8 estimates, excluding one tie). In all seasons, the MCP (all r2 < 0.26 and P ≥ 0.20) and FK (all r2 < 0.27 and P ≥ 0.19) estimates were indepen-dent of number of fixes.

Autocorrelation.—Autocorrelation values (Schoener’s t2/r2 statistic) varied from 0.62-2.80 for individual home range estimates, with means for the six cells (2 species × 3 seasons) ranging from 1.40 to 2.02 (Table 1). Schoener values substantially <2.0 are indicative of time dependence between successive relocations (Swihart and Slade, 1985). The ANOVA showed that autocorrelation values were simi-lar for the two species and three seasons, with no interac-tion (all P ≥ 0.52; all partial η2 < 0.13; Table 2). Likewise, independent t-tests for each of the three seasons revealed no species differences (all P ≥ 0.65) and effect sizes were small (all Cohen’s d ≤ 0.27). Autocorrelation values during each of the three seasons were independent of both MCP (all r2 < 0.02, P ≥ 0.65) and FK (all r2 < 0.14, P ≥ 0.26), and number of fixes (all r2 < 0.10, P ≥ 0.32).

Movements.—Mean daily movements of individual snakes varied from 2.93-37.18 m/d during the active sea-son to 0.85-3.69 m/d during the winter (Table 1). From the ANOVA, the significant main effect of species (P = 0.037, partial η2 = 0.39) indicated that C. o. helleri moved greater distances on average than C. ruber (Table 2). The differ-ences were most evident during the active seasons, with C. o. helleri moving 2.8-fold further in 2003 and 1.9-fold further in 2004 (using species means in Table 1). The main effect of season was also significant (P = 0.001, partial η2 = 0.41), with snakes moving considerably less during the winter compared to the active seasons (C. ruber = 14.7% and C. o. helleri = 13.1% of their respective active-season movements, calculated from species means in Table 1 aver-aged for the two active seasons). There was no interaction between season and species (Table 2). When the two spe-cies were compared for each season by t-tests, mean daily

movements differed for each of the two active seasons (ac-tive 2003: t10 = 2.52, P = 0.031, Cohen’s d = 1.45; active 2004: t10 = 2.48, P = 0.032, Cohen’s d = 1.43), but not for the winter season (P = 0.12), though the effect size was substantial (Cohen’s d = 1.13).

Movements were positively correlated with MCP in one season (active 2003: r2 = 0.62, P = 0.002), but not in the others (both r2 < 0.18 and P ≥ 0.30). Movements were similarly associated with FK in one season (active 2003: r2 = 0.64, P = 0.002), but not in the others (both r2 < 0.26 and P ≥ 0.11). Movements were independent of number of fixes in all seasons (all r2 < 0.26 and P ≥ 0.09), but were weakly positively associated with autocorrelation in one season (active 2004: r2 = 0.33, P = 0.049; other seasons: r2 < 0.21 and P ≥ 0.25). Although not amenable to statistical analysis, monthly patterns of activity (Fig. 2) revealed peaks in both spring and late summer/fall, with relatively less activity oc-curring during the summer months. The late summer/fall peak in movement appeared to be more pronounced for C. o. helleri than for C. ruber.

Individuals of both species moved on occasion during the winter, demonstrating a lack of overwintering single-site fidelity. Neither species used communal hibernacula.

Table 2. Summary of analysis of variance (ANOVA) results for home range estimates (100% minimum convex polygon, MCP; 95% fixed kernel, FK), mean daily movements, and autocorrelation (t2/r2) for two species of rattlesnake over three seasons (see Table 1). Results include degrees of freedom (df), F-value, probability (P), and effect size (adjusted partial η2).

Dependent Season Species InteractionVariable df F P η2 df F P η2 df F P η2

________________________________________________________________________________________________________________

MCP 2,8 8.89 0.041 0.47 1,4 2.08 0.223 0.29 2,8 1.66 0.250 0.25FK 2,8 5.39 0.033 0.43 1,4 2.96 0.160 0.32 2,8 1.97 0.201 0.25Movements 2,12 15.35 0.001 0.41 1,6 7.14 0.037 0.39 2,12 2.18 0.156 0.20t2/r2 2,8 0.51 0.621 0.11 1,4 0.49 0.524 0.11 2,8 0.62 0.562 0.13

Fig. 1

Figure 1. Mean (+ 1 SE) 95% fixed kernel estimates of home range size for adult male Red Diamond Rattlesnakes (Crotalus ruber) and Southern Pacific Rattlesnakes (C. oreganus helleri) during three seasons at Chino Hills State Park, California. Sample sizes for each mean are within parentheses.

358

Mating phenology.—At our study site, we observed distinctly different use of mating seasons in the two spe-cies (Table 3). Crotalus ruber exhibited accompaniment, courtship, and copulation only during spring (2 February-7 April), whereas C. o. helleri exhibited sexual behaviors dur-ing both spring (16 February-24 April) and late summer/fall (11 September-3 October). For total observations (accom-paniment and copulations), the asymmetry between species and season was significant (Cramer’s V = 0.41, P = 0.013). When additional data from existing literature were added to our observations (Table 3), the asymmetry was even stron-ger (Cramer’s V = 0.53, P < 0.001).

Discussion

We expected to see seasonal differences in home range size and movements, with both species occupying larger home ranges and moving greater distances during the ac-tive season than during the winter. However, we had no a priori expectations of finding the species differences that were detected. In the sections that follow, we emphasize these species differences, but also render comparisons to other studies when relevant.

Home range size.—Although we did not detect a sta-tistical difference in home range size between the two spe-cies, effect sizes were large enough to suggest that a dis-parity exists, with C. o. helleri occupying two- to five-fold larger home ranges than C. ruber at our study site. Unfortu-nately, the relatively small sample size resulting from high levels of predation on C. o. helleri (see Methods) reduced the power of our statistical analyses, but practical signifi-cance (effect size) is often more meaningful, as it can be interpreted apart from sample size effects (Cohen, 1988). During winter, the two species likely occupied similarly-sized home ranges, and these were much smaller than those used during the active season. Although we did not track females during this study, C. ruber females use signifi-cantly smaller home ranges than males (Greenberg, 2002; Brown et al., this volume). We attribute species differences in home range size to the different preferred habitats, as discussed in the next section.

A comparison of 100% MCP estimates from three lo-cations in southern California suggests that coastal popula-

tions of C. ruber occupy smaller home ranges than those of desert populations. At our coastal site, the seven adult males averaged 2.4 ha, with individuals occupying 0.3-4.5 ha (based on 6-40 fixes during the 9-month active season over 2 yr; Table 1). Five telemetered adult males at a coastal San Diego County location averaged 2.8 ha, with individu-als ranging from 1.1-4.4 ha (based on 18-79 unique fixes during up to a 4-yr duration; Brown et al., this volume). In contrast to coastal populations, five telemetered adult males at a desert location in Riverside County averaged 25.8 ha, with individuals ranging from 7.2-52.5 ha (based on an unknown number of fixes over 381-1,000 d; Greenberg, 2002). Intraspecific variation in home range size could be attributed to differential availability and distribution of resources in coastal and desert environments (Gregory et al., 1987). One might expect that reliance on communal hibernacula would increase home range size, with snakes often dispersing great distances during the active season to reduce interspecific competition (Reinert and Zappalorti, 1988; Martin, 1992; Brown, 1993; Jørgensen et al., this volume; see further comments below). However, whereas snakes at our site and at the desert Riverside County loca-tion (Greenberg, 2002) did not use communal overwinter-ing sites, those at the coastal San Diego County site did so, and their home ranges were relatively small (Brown et al., this volume). Unfortunately, we cannot compare our results from C. o. helleri to other locations due to the absence of such studies.

In our study, MCP and FK estimates showed close cor-respondence (with CR2 being the one exception in Table 1). Although both home range estimators are sample size-de-pendent (MCP increasing and FK decreasing with increas-ing sample size; Seaman et al., 1999) and FK estimates are especially sensitive to autocorrelation (Powell, 2000), we found no overt influence of number of fixes or autocorrela-tion on our estimates. The consequences of autocorrelation are frequently ignored in studies of home range size, though the sedentary nature of snakes, and reptiles in general, can often result in autocorrelation (Row and Blouin-Demers, 2006; Figueroa et al., this volume).

Movements and mating phenology.—Crotalus ruber not only used a smaller home range size than C. o. helleri, but also moved significantly less. We hypothesize two as-

E. A. Dugan, A. Figueroa, and W. K. Hayes

Table 3. Number of sexual interactions observed in Red Diamond (Crotalus ruber) and Southern Pacific (C. oreganus helleri) Rattlesnakes, including male-female pairs interacting (accompaniment, sometimes including courtship) or copulating at our study site during spring and late summer/fall. Numbers in parentheses represent sum of our observations and reports from existing literature.a

Sexual Spring Late Summer/FallInteractions C. ruber C. o. helleri C. ruber C. o. helleri________________________________________________________________________________________________________________

Accompaniment 10 (19) 8 (8) 0 (0) 3 (3)Copulation 9 (27) 5 (6) 0 (0) 2 (5)________________________________________________________________________________________________________________

Totals 19 (46) 13 (14) 0 (0) 5 (8)

aSources: Linsdale (1932), Armstrong and Murphy (1979), Klauber (1972), Grismer (2002), Brown et al. (this volume).

359Ecology of Red Diamond and Southern Pacific Rattlesnakes

pects of the biology of each species that likely contributed to the differences observed in active-season movements: habi-tat use and the timing and frequency of mating seasons.

The availability and distribution of preferred habitat have been shown to affect both annual (Reinert, 1993) and seasonal (Marshall et al., 2006) movements of rattlesnakes. At our site, C. ruber and C. o. helleri exhibited significant differences in habitat use (Dugan and Hayes, 2005, unpubl. data). Preferred habitat of C. ruber at our site—cactus pri-marily, and chaparral—is sparsely distributed and restricted to a series of south-facing hillsides. Movements of C. ru-ber throughout the study period were primarily confined to moves between and within clusters of Opuntia cactus patches. Several individuals spent virtually an entire active season within a single cactus patch. In contrast, preferred habitat of C. o. helleri—grassland and riparian—is widely and more continuously distributed throughout the site. The greater mean distances moved annually by C. o. helleri may be associated with the less-restricted abundance of grass-land and riparian habitats.

Reproductive condition also dramatically affects the movements of both male and female rattlesnakes. Dur-ing mating season, male rattlesnakes search extensively for females to mate (e.g., Duvall et al., 1992; Secor, 1992; Aldridge and Brown, 1995; Prival et al., 2002; Sealy, 2002; Ashton, 2003; Marshall et. al, 2006; Waldron et al., 2006a; Jellen et al., 2007; Cardwell, 2008; Brown et al., this vol-ume; Goode et al., this volume). Female rattlesnakes, in contrast, do not actively search for mates (Duvall and Schuett, 1997), and typically exhibit smaller home ranges and reduced movements, with gravid individuals being most sedentary. At our study site, male C. o. helleri moved signif-icantly greater distances than male C. ruber. The difference may result, in part, from the different mating phenology of the two species. Crotalus ruber at our site appears to mate exclusively in the spring (Table 3). A comprehensive review of the literature suggests that this is a range-wide phenom-enon (Klauber, 1972; Goldberg, 1999; Aldridge and Duvall, 2002; Greenberg, 2002; Grismer, 2002; Campbell and La-mar, 2004; Brown et al., this volume), as we were unable to locate any published records of reproductive activity in C. ruber outside of the well-documented spring mating period. Crotalus o. helleri, in contrast, mates during both spring and late summer/fall at our site (Table 3), and prob-ably does so throughout its range (Aldridge, 2002). Males of both species at our study site exhibited increased move-ments during the spring mating season, but late summer/fall movements by C. o. helleri appeared to be greater than those of C. ruber (Fig. 2). Thus, the finding that C. o. helleri moves significantly greater distances than C. ruber during the active season may reflect the additional late summer/fall mating season of C. o. helleri.

The reason for the marked difference between C. ruber and C. o. helleri in the timing and frequency of mating sea-sons remains unclear. A similar disparity exists for sympat-ric C. atrox (late summer/fall and spring mating) and C. mo-

lossus (spring mating) in Arizona, with C. atrox exhibiting two corresponding peaks in testosterone and C. molossus only one (Schuett et al., 2005). Thus, the differences may be physiologically based if the behaviors are hormonally depen-dent, but clearly do not derive from ambient environmental factors (e.g., photoperiod, temperature, precipitation) and more likely relate to evolutionary history (Aldridge and Du-vall, 2002; Schuett et al., 2002, 2005). However, because re-productive isolation between sympatric rattlesnakes almost certainly results from pheromone differentiation (Shine et al., 2002, 2004), and the costs borne by males to locate fe-males by following pheromone trails can be high (Aldridge and Brown, 1995; Aldridge and Duvall, 2002; McGowan and Madison, this volume), temporal segregation of mating seasons may have originated from competitive mechanisms if the costs for discriminating conspecific from heterospe-cific female pheromone trails are high. Such costs might be expected to be higher during the late summer/fall mating season, as both C. ruber and C. o. helleri at our study site of-ten overwinter in relatively close proximity on south-facing slopes (Dugan and Hayes, unpubl. data), where mate loca-tion would be facilitated in spring.

Fig. 2

Figure 2. Mean daily movements per month (numbered January-December) of adult male Red Diamond Rattlesnakes (Crotalus ruber; dashed lines) and Southern Pacific Rattlesnakes (C. oreganus helleri; solid lines) during each of two years. For each mean, N = 2-6.

360

The annual movements of rattlesnakes vary substan-tially across species and may relate to intraspecific com-petition. Some populations are clearly migratory, typically moving substantial distances between their communal hi-bernaculas and their active-season home ranges (e.g., Mac-artney et al., 1988; Brown, 1993; Duvall and Schuett, 1997; Jørgensen et al., this volume). Migratory populations gen-erally inhabit regions that experience extreme winter tem-peratures with limited suitable overwintering sites (for an exception, see Harvey and Weatherhead, 2006). However, rattlesnake populations in milder climates, such as those at our study site, frequently hibernate solitarily without strong site fidelity and lack annual unidirectional movements (e.g., Secor, 1992; Greenberg, 2002). Home range size may also be smaller in non-migratory populations. We concur with Greenberg (2002) that differences in movements and spatial use by individuals in these populations likely arise from dif-ferences in competition. Snakes at communal hibernacula face higher levels of intraspecific (and often interspecific) competition and, therefore, must disperse to a greater extent than snakes that hibernate solitarily.

Niche partitioning.—Niche separation between close-ly-related sympatric species can be achieved via differences in food (type or size) or in use of spatial (macrohabitat or microhabitat), temporal (diel or seasonal activity patterns), or thermal resources (Schoener, 1974; Saint Girons, 1978). Our findings suggest that sympatric populations of C. ruber and C. o. helleri, like other sympatric rattlesnakes (Pough, 1966; Reynolds and Scott, 1982; Mendelson and Jennings, 1992; Beck, 1995; Waldron et al., 2006b; Steen et al., 2007) and vipers (Reinert, 1984; Luiselli, 2006a,b; Luiselli et al., 2007) studied to date, primarily partition the habitat to re-duce interspecific competition. However, unlike other sym-patric rattlesnakes investigated thus far, C. ruber and C. o. helleri also differ in their use of spatial and temporal re-sources, though probably not as a direct result of competi-tive mechanisms. Although diet may vary among sympatric rattlesnakes (e.g., Clark, 2002; Holycross et al., 2002), in-cluding between C. ruber and C. o. helleri at our study site (Dugan, unpubl. data), rattlesnakes are often opportunistic feeders (within broader categories of preferred prey types, such as lizards and mammals; e.g., Klauber, 1972; Clark, 2002; Holycross et al., 2002; Campbell and Lamar, 2004; Avila-Villegas et al., 2007), and dietary differences may re-late primarily to prey availability in the different preferred habitats and, secondarily, to size differences (gape-limited constraints) between adults (Reynolds and Scott, 1982). We imagine that diet might be partitioned more so than habitat among some sympatric rattlesnakes that differ substantially in adult size and coexist in environments with limited hab-itat variability (e.g., C. atrox and C. cerastes in Creosote [Larrea tridentata] desert flats). Apart from differences in mating seasons, we found no evidence that C. ruber and C. o. helleri further partition temporal resources. In terms of diel activity, we found both species active primarily during morning and evening hours, with some shift to nocturnal

E. A. Dugan, A. Figueroa, and W. K. Hayes

activity during hot weather (Dugan, unpubl. data). In terms of seasonal activity, we found C. ruber to be relatively sed-entary compared to C. o. helleri, but the timing and dura-tion of the active season were similar. Although we have not yet examined body temperature data, we are doubtful that thermal resources are partitioned between these two species. Perhaps further study of other sympatric rattlesnakes will re-veal examples wherein food, temporal, or thermal resources are substantially partitioned to facilitate coexistence.

Acknowledgments

All research was carried out under permits issued to the senior author by the California Department of Fish and Game and by Chino Hills State Park (CHSP). We thank ranger Bart Grant of CHSP for his assistance in our re-search. We received significant technical and field support from numerous individuals, including Mike Cardwell, Brian McGurty, Chris Rodriguez, Sean Bush, and Kent Beaman. The Loma Linda University Department of Earth and Bio-logical Sciences provided financial support for the studies. This project was approved by the Loma Linda University Institutional Animal Care and Use Committee and was con-ducted under a permit issued by the California Department of Fish and Game.

Literature Cited

Aldridge, R. D. 2002. The link between mating season and male reproductive anatomy in the rattlesnakes Crotalus viridis oreganus and Crotalus viridis helleri. J. Herpe-tol. 36:295-300.

———, and W. S. Brown. 1995. Male reproductive cy-cle, age at maturity, and cost of reproduction in the Timber Rattlesnake (Crotalus horridus). J. Herpetol. 29:399-407.

———, and D. Duvall. 2002. The evolution of the mat-ing season in the pitvipers of North America. Herpetol. Monogr. 16:1-25.

Armstrong, B. L., and J. B. Murphy. 1979. The natural history of Mexican rattlesnakes. Univ. Kansas Publ., Mus. Nat. Hist. 51:1-88.

Ashton, K. G. 2003. Movements and mating behaviour of adult male Midget Faded Rattlesnakes, Crotalus oreganus concolor, in Wyoming. Copeia 2003:190-194.

———, and A. de Queiroz. 2001. Molecular systemat-ics of the Western Rattlesnake, Crotalus viridis (Vi-peridae), with comments on the utility of the D-loop in phylogenetic studies of snakes. Mol. Phylogen. Evol. 21:176-189.

Avila-Villegas, H., M. Martins, and G. Arnaud. 2007. Feeding ecology of the endemic Rattleless Rattlesnake, Crotalus catalinensis, of Santa Catalina Island, Gulf of California, Mexico. Copeia 2007:80-84.

Beaman, K. R., and E. A. Dugan. 2006. Crotalus ruber (Red Diamond Rattlesnake). Cat. Am. Amph. Rept. 840:1-17.

361

Beck, D. 1995. Ecology and energetics of three sympatric rattlesnake species in the Sonoran Desert. J. Herpetol. 29:211-223.

Brown, W. S. 1993. Biology, status, and management of Timber Rattlesnakes (Crotalus horridus): a guide for conservation. Herpetol. Circ. No. 22, Society for the Study of Amphibians and Reptiles, Lawrence, Kansas.

Campbell, J. A., and W. W. Lamar. 2004. The Venomous Reptiles of the Western Hemisphere. 2 vols. Cornell University Press, Ithaca, New York.

Cardwell, M. D. 2008. The reproductive ecology of Mo-have Rattlesnakes. J. Zool. 274:65-76.

Clark, R. 2002. Diet of the Timber Rattlesnake, Crotalus horridus. J. Herpetol. 36:494-499.

Cohen, J. 1988. Statistical Power Analysis for the Behav-ioral Sciences. 2nd ed. Erlbaum, Hillsdale, New Jersey.

Conover, W. J. 1999. Practical Nonparametric Statistics. 3rd ed. John Wiley and Sons, Inc., New York, New York.

Diller, L. V., and R. L. Wallace. 2002. Growth, repro-duction, and survival in a population of Crotalus virid-is oreganus in north central Idaho. Herpetol. Monog. 16:26-45.

Dugan, E. A., and W. K. Hayes. 2005. Comparative ecol-ogy of Red Diamond (Crotalus ruber) and Southern Pacific (Crotalus helleri) Rattlesnakes in southern Cal-ifornia. Pg. 27 in Program and Abstracts of the Biology of the Rattlesnakes Symposium. Loma Linda Univer-sity, Loma Linda, California.

Duvall, D., and G. W. Schuett. 1997. Straight-line movements and competitive mate searching in Prairie Rattlesnakes, Crotalus viridis viridis. Anim. Behav. 54:329-334.

———, S. J. Arnold, and G. W. Schuett. 1992. Pitviper mating systems: ecological potential, sexual selection, and microevolution. Pp. 169-184 in J. A. Campbell and E. D. Brodie Jr. (eds.). Biology of the Pitvipers. Selva Press, Tyler, Texas.

Goldberg, S. R. 1999. Reproduction in the Red Diamond Rattlesnake in California. California Fish and Game 85:177-180.

Goodman, R. H. 1997. The biology of the Southwestern Pond Turtle (Clemmys marmorata pallida) in the Chi-no Hills State Park and the west fork of the San Gabriel River. M.S. Thesis, California State University, Fuller-ton, California.

Greenberg, D. B. 2002. The ecology of movement and site selection in desert rattlesnakes (Crotalus mitchellii and Crotalus ruber) of the southwestern United States. Ph.D. Dissertation, University of California, Santa Barbara, California.

Gregory, P. T., J. M. Macartney, and K. W. Larsen. 1987. Spatial patterns and movements. Pp. 366-395 in R. A. Seigel, J. T. Collins, and S. S. Novak (eds.), Snakes: Ecology and Evolutionary Biology. McGraw-Hill, Inc., New York, New York.

Ecology of Red Diamond and Southern Pacific Rattlesnakes

Grismer, L. L. 2002. Amphibians and Reptiles of Baja Cali-fornia Including its Pacific Islands and the Islands in the Sea of Cortés. University of California Press, Berkeley, California.

Hardy, D. L., and H. W. Greene. 1999. Surgery on rattle-snakes in the field for implantation of transmitters. So-noran Herpetol. 12:25-27.

Harvey, D. S., and P. J. Weatherhead. 2006. Hibernation site selection by Eastern Massasauga Rattlesnakes (Sis-trurus catenatus catenatus) near their northern range limit. J. Herpetol. 40:66-73.

Holycross, A. T., C. W. Painter, D. B. Prival, D. E. Swann, M. J. Schroff, T. Edwards, and C. R. Schwalbe. 2002. Diet of Crotalus lepidus klauberi (Banded Rock Rattlesnake). J. Herpetol. 36:589-597.

Hovey, F. W. 1999. The Home Ranger. British Columbia Forest Service Research Branch, Revelstoke, British Columbia, Canada.

Jellen, B. C., D. B. Shepard, M. J. Dreslik, and C. A. Phillips. 2007. Male movement and body size affect mate acquisition in the Eastern Massasauga (Sistrurus catenatus). J. Herpetol. 41:451-457.

Jennings, M. R., and M. P. Hayes. 1994. Amphibian and Reptile Species of Special Concern in California. Cali-fornia Department of Fish and Game, Rancho Cordova, California.

Keller, T. K. 1992. Relationship of plant ecology and hy-drology in Aliso Canyon riparian zone, Chino Hills, southern California. M.S. Thesis, California State Uni-versity, Fullerton, California.

Kie, J. G., J. A. Baldwin, and C. J. Evans. 1994. Calhome: Home Range Analysis Program. U.S. Forest Service Pa-cific Southwest Research Station, Fresno, California.

Klauber, L. M. 1972. Rattlesnakes: Their Habits, Life His-tories, and Influence on Mankind 2 vols. 2nd ed. Univer-sity of California Press, Berkeley, California.

Laver, P. N., and M. J. Kelly. 2008. A critical review of home range studies. J. Wildl. Mgmt. 72:290-298.

Linsdale, J. M. 1932. Amphibians and reptiles from Lower California. Univ. Calif. Publ. Zool. 38:345-386.

Luiselli, L. 2006a. Resource partitioning and interspecific competition in snakes: the search for general geographi-cal and guild patterns. Oikos 114:193-211.

———. 2006b. Site occupancy and density of sympatric Gaboon Viper (Bitis gabonica) and Nose-horned Viper (Bitis nasicornis). J. Trop. Ecol. 22:555-564.

———, E. Filippi, and E. Di Lena. 2007. Ecological re-lationships between sympatric Vipera aspis and Vipera ursinii in high-altitude habitats of central Italy. J. Her-petol. 41:378-384.

Macartney, J. M., P. T. Gregory, and K. W. Larsen. 1988. A tabular survey of data on movements and home ranges of snakes. J. Herpetol. 22:61-73.

Marshall, J. C., Jr., J. V. Manning, and B. A. Kings-bury. 2006. Movement and macrohabitat selection of the Eastern Massasauga in a fen habitat. Herpetologica 62:141-150.

362

Martin, W. H. 1992. Phenology of the Timber Rattlesnake (Crotalus horridus) in an unglaciated section of the Appalachian Mountains. Pp. 169-184 in J. A. Camp-bell and E. D. Brodie, Jr. (eds.), Biology of the Pitvi-pers. Selva, Tyler, Texas.

Mendelson, J. R., III, and W. B. Jennings. 1992. Shifts in the Relative Abundance of Snakes in a Desert Grass-land. J. Herpetol. 26:38-45.

Mertler, C. A., and R. A. Vannatta. 2004. Advanced and Multivariate Statistical Methods: Practical Appli-cation and Interpretation. 3rd ed. Pyrczak Publishing, Los Angeles, California.

Pianka, E. 1981. Competition and niche theory. Pp. 167-196 in R. May (ed.), Theoretical Ecology. Principles and Applications. Blackwell, Oxford, United Kingdom.

Pough, F. H. 1966. Ecological relationships of rattlesnakes in southeastern Arizona with notes on other species. Copeia 1966:676-683.

Powell, R. A. 2000. Animal home ranges and territories and home range estimators. Pp. 65-110 in L. Boitani and T. K. Fuller (eds.), Research Techniques in Animal Ecology. Columbia University Press, New York, New York.

Prival, D. B., M. J. Goode, D. E. Swann, C .R. Schwalbe, and M. J. Schroff. 2002. Natural history of a northern population of Twin-spotted Rattlesnakes, Crotalus pri-cei. J. Herpetol. 36:598-607.

Reinert, H. K. 1984. Habitat separation between sympat-ric snake populations. Ecology 65:478-486.

———. 1993. Habitat selection in snakes. Pp. 201-240 in R. A. Seigel and J. T. Collins (eds.), Snakes: Ecology and Behavior. McGraw-Hill, Inc., New York, New York.

———, and D. Cundall. 1982. An improved surgical implantation method for radio-tracking snakes. Copeia 1982:702-705.

———, and R. T. Zappalorti. 1988. Timber Rattle-snakes (Crotalus horridus) of the Pine Barrens: their movement patterns and habitat preference. Copeia 1988:964-978.

Reynolds, R. P., and N. J. Scott, Jr. 1982. Use of a mammalian resource by a Chihuahuan snake commu-nity. Pp. 99-118 in N. J. Scott, Jr. (ed.), Herpetologi-cal Communities. U.S. Dept. Interior, Fish and Wildl. Serv., Wild. Res. Rep. 13.

Row, J. R., and G. Blouin-Demers. 2006. Kernels are not accurate estimators of home-range size for herpetofau-na. Copeia 2006:797-802.

Saint Girons, H. 1978. Thermorégulation comparé de vipères d’Europe. Etude biotélémétrique. Rev. Ecol. (Terre et Vie) 32:417-439.

Sampson, G. T. 1985. A flora of Chino Hills State Park. M.A. Thesis, California State University, Fullerton.

Schoener, T. W. 1974. Resource partitioning in ecological communities. Science 185:27-39.

Schuett, G. W., S. L. Carlisle, A. T. Holycross, J. K. O’Leile, D. L. Hardy, Sr., E. A. Van Kirk, and W.

J. Murdoch. 2002. Mating system of male Mojave Rattlesnakes (Crotalus scutulatus): seasonal timing of mating, agonistic behavior, spermatogenesis, sexual segment of the kidney, and plasma sex steroids. Pp. 515-532 in G. W. Schuett, M. Höggren, M. E. Douglas, and H. W. Greene, Biology of the Vipers. Eagle Moun-tain Publishing, Eagle Mountain, Utah.

———, D. L. Hardy, Sr., H. W. Greene, R. L. Earley, M. S. Grober, E. A. Van Kirk, and W. J. Murdoch. 2005. Sympatric rattlesnakes with contrasting mating systems show differences in seasonal patterns of plas-ma sex steroids. Anim. Behav. 70:257-266.

Sealy, J. B. 2002. Ecology and behavior of the Timber Rattlesnake (Crotalus horridus) in the Upper Piedmont of North Carolina: identified threats and conservation recommendations. Pp. 561-578 in G. W. Schuett, M. Höggren, M. E. Douglas, and H. W. Greene, Biology of the Vipers. Eagle Mountain Publishing, Eagle Moun-tain, Utah.

Seaman, D. E., J. J. Millspaugh, B. J. Kernohan, G. C. Brundige, K. J. Raedeke, and R. A. Gitzen. 1999. Effects of sample size on kernel home range estima-tors. J. Wildl. Mgmt. 63:739-747.

Secor, S. M. 1992. A preliminary analysis of the movement and home range size of the Sidewinder, Crotalus cer-astes. Pp. 169-184 in J. A. Campbell and E. D. Brodie, Jr. (eds.), Biology of the Pitvipers. Selva, Tyler, Texas.

Seigel, R. A., and M. A. Pilgrim. 2002. Long-term chang-es in movement patterns of Massasaugas (Sistrurus catenatus). Pp. 405-512 in G. W. Schuett, M. Höggren, M. E. Douglas, and H. W. Greene (eds.), Biology of the Vipers. Eagle Mountain Publishing, Eagle Mountain, Utah.

Shine, R., B. Phillips, H. Waye, M. Lemaster, and R. T. Mason. 2004. Species-isolating mechanisms in a mating system with male mate choice (garter snakes, Thamnophis spp.). Can. J. Zool. 82:1091-1098.

———, R. N. Reed, S. Shetty, M. LeMaster, and R. T. Mason. 2002. Reproductive isolating mechanisms between two sympatric sibling species of sea-snakes. Evolution 56:1655–1662.

SPSS. 2003. SPSS for Windows, version 12.0. SPSS, Inc., Chicago, Illinois.

Stebbins, R.C. 2003. Western Reptiles and Amphibians, 3rd ed. Houghton Mifflin, Boston, Massachusetts.

Steen, D. A., L. L. Smith, L. M. Conner, J. C. Brock, and S. K. Hoss. 2007. Habitat use of sympatric rattle-snake species within the Gulf Coastal Plain. J. Wildl. Mgmt. 71:759-764.

Swihart, R. K., and N. A. Slade. 1985. Influence of sampling interval on estimates of home-range size. J. Wildl. Mgmt. 49:1019-1025.

Toft, C. A. 1985. Resource partitioning in amphibians and reptiles. Copeia 1985:1-21.

Vitt, L. J. 1987. Communities. Pp. 335-365 in R. A. Seigel, J. T. Collins, and S. S. Novak (eds.), Snakes: Ecology

E. A. Dugan, A. Figueroa, and W. K. Hayes

363

and Evolutionary Biology. MacMillan Publishing Co., New York, New York.

Waldron, J. L., J. D. Lanham, and S. H. Bennett. 2006a. Using behaviorally-based seasons to investigate Cane-brake Rattlesnake (Crotalus horridus) movement pat-terns and habitat selection. Herpetologica 62:389-398.

———, S. H. Bennett, S. M. Welch, M. E. Dorcas, J. D. Lanham, and W. Kalinowsky. 2006b. Habitat speci-

ficity and home-range size as attributes of species vul-nerability to extinction: a case study using sympatric rattlesnakes. Anim. Conserv. 9:414-420.

Walter, G. H. 1991. What is resource partitioning? J. The-or. Biol. 150:137–143.

Ecology of Red Diamond and Southern Pacific Rattlesnakes

364 E. A. Dugan, A. Figueroa, and W. K. Hayes